The biological activity of angiotensin II, which has been modified by arginylation in the reaction catalyzed by arginyl-tRNA-protein transferase, will be compared with that of the unmodified octapeptide. The reaction catalyzed by the soluble fraction of rabbit liver in which radioactivity from iodinated tyrosines and thyroid hormones is incorporated into protein will be investigated. It is known that the corresponding alpha-keto acids generated by transamination are intermediates and these compounds will be used as substrates to detect protein factors other than the transaminase which may be required in the incorporating system.